1. Field of the Invention
The present invention generally relates to an information processing device, and more particularly to an information processing device which is connected to an interface system such as a keyboard interface system and is supplied with power from an upper device to which the interface system is connected and power from an external power source.
In the present specification, a description will mainly be given of a bar code reader connected to a keyboard interface. However, the present invention is not limited to such a bar code reader but includes an information processing device which has a plurality of power supply systems and communicate with a device connected to the information processing device.
2. Description of the Related Art
A system having a bar code reader connected to a personal computer has been widely used to input information represented by a bar code. A bar code reader of so-called touch type is known.
Generally, the bar code reader performs a bar code read operation in which a light is projected onto a bar code and part of the light reflected by the bar code is received. The touch-type bar code reader is equipped with a light-emitting device (LED) as a light source and a two-dimensional CCD (Charge-Coupled Device) serving as a light-receiving element. The operator brings a read window of the touch-type bar code reader into contact with the bar code to be read. Hence, the bar code is illuminated by the light emitted from the LED, and the light reflected by the bar code is received by the CCD, so that information represented by the bar code can be read.
The touch-type bar code reader has a simple, light, easily operable, and less-expensive structure because it does not have any mechanism for deflecting the light for scanning. Further, the touch-type bar code reader does not have any demodulator which demodulates read bar code data and any mechanism which mechanically drives a component. Thus, the touch-type bar code reader has been widely used due to the above advantages thereof.
FIG. 1 is a block diagram of a system structure having such a bar code reader connected between a personal computer and a keyboard. Particularly, FIG. 1 is a part of the system structure shown in FIG. 1, the above part being involved with a signal transmit/receive and a power supply. The system shown in FIG. 1 includes a personal computer 11, a keyboard 12, and a bar code reader 13, which are respectively equipped with controllers 11a, 12a and 13a for controlling the respective operations thereof.
The personal computer 11, the bar code reader 13 and the keyboard 12 are connected through an interface cable 14, which includes a transmission line 14b and a power supply line 14a. The transmission line 14b of the interface cable 14 is used to transmit and receive data, and the power supply line 14a thereof is used to supply electricity to the keyboard 12 from the personal computer 11.
The controllers 11a, 12a and 13a have respective input and output terminals to which the transmission line 14b of the interface cable 14 is connected. Drivers are provided to the output terminals of the controllers 11a, 12a and 13a.
Electricity for operating the keyboard 12 is supplied from the personal computer 11 via the power supply line 14a. The power supply voltage is denoted by V.sub.PC. The touch-type bar code reader 13 consumes a small amount of power. Hence, in the system shown in FIG. 1, the power supply voltage V.sub.PC of the personal computer 11 is applied to the bar code reader 13 as well as the keyboard 12 so that electricity for operating the bar code reader 13 is supplied from the personal computer 11 via the power supply line 14a.
A signal which is input when a key of the keyboard 12 is operated or depressed and bar code data input by the bar code reader 13 are transmitted to the personal computer 11 over the transmission line 14b. An instruction can also be transmitted to the keyboard 12 and the bar code reader 13 via the transmission line 14b.
Recently, there has been a demand to efficiently execute the bar code input process of the bar code reader. An advanced bar code reader having high performance is known in which it employs a scanner such as a polygon mirror. The scanner is driven or rotated by a stepping motor which serves as a driving source. The polygon mirror has mirror surfaces by which a light beam emitted from a light source such as a laser diode is reflected so that the bar code is scanned.
In the touch-type bar code reader, the demodulation process for the read bar code data is processed by an upper computer such as a personal computer. However, in this case, the personal computer has an increased load of processing. In order to reduce the load of the personal computer, a modified touch-type bar code reader has been proposed which is equipped with the demodulation circuit which demodulates the bar code data. In this case, the personal computer does not execute the demodulation process.
As the performance of the bar code reader is improved, an increased amount of electricity is consumed therein. The structure shown in FIG. 1 increases the load of the power source of the personal computer 11 because the bar code reader 13 is supplied with electricity from the personal computer 11. In some cases, a sufficient amount of energy to drive the bar code reader 13 cannot be supplied thereto.
In order to avoid the above problem, an external power source is provided separately from the power source of the personal computer 11. The external power source is externally provided to and connected to the bar code reader 13. An AC adapter is an example of such an external power source.
However, another problem will occur when such an external power source is provided. Generally, electronic devices such as bar code readers and personal computers employ CMOS (Complementary Metal Oxide Semiconductor) circuits because the CMOS circuits consume a relatively small amount of energy. As is well know, the CMOS circuits has a problem so-called "latch up".
FIG. 2 is a circuit diagram of a CMOS circuit, which includes an n-type MOS transistor and a p-type MOS transistor, the drains of which transistors are connected together. A power supply voltage V.sub.DD is applied to the CMOS circuit. A symbol V.sub.IN denotes an input signal, and a symbol V.sub.OUT denotes an output signal. The CMOS circuit shown in FIG. 2 functions as an inverter, which consumes a relatively small amount of energy and is thus used widely.
The latch-up occurs in the CMOS circuit shown in FIG. 2 if the voltage of the input signal V.sub.IN is greater than the power supply voltage V.sub.DD.
FIG. 3 is a cross-sectional view of the CMOS circuit shown in FIG. 2. A parasitic p-n-p bipolar transistor .alpha..sub.1 and a parasitic n-p-n bipolar transistor .alpha..sub.2 are formed, as indicated by the broken line in FIG. 3. The above two parasitic bipolar transistors connected in the way indicated by the broken lines form a thyristor. If the thyristor starts to operate, a current continuously flows from the power supply V.sub.DD to the ground. This phenomenon is called latch-up. In order to release the circuit from the latch-up state, it is necessary to turn of the power supply voltage V.sub.DD. In some cases, an excessive current flows due to the latch-up, and the circuit may be broken.
The latch-up causes the following problem in a state in which electricity is supplied to either the personal computer 11 or the bar code reader 12 only a signal is transferred over the transmission line 14b. In this-state, the latch-up occurs in a CMOS circuit provided in the device supplied with no electricity. The above problem will be described with reference to FIG. 4, in which parts that are the same as those shown in FIG. 1 are given the same reference numbers.
Referring to FIG. 4, electricity is supplied to the personal computer 11, while no electricity is supplied to the bar code reader 13. In this case, no voltage is supplied from an AC adapter 15, so that a voltage of 0 V is supplied to the controller 13a. When a signal having a high level of +5 V is output to the transmission line 14b from the personal computer 11. In this case, a voltage of +5 V is applied to the input terminal of the controller 13a. Hence, the voltage applied to the input terminal of the controller 13a is higher than the power supply voltage applied thereto. This causes the latch-up in a CMOS circuit in the controller 13a.
The latch-up will not frequently occur in the configuration shown in FIG. 1 because the bar code reader 13 is supplied with electricity from the personal computer 11. Hence, when the V.sub.PC power supply ON/OFF states of the personal computer 11 and the bar code reader 13 cooperate with each other. However, as has been described previously, the configuration shown in FIG. 1 has a problem in terms of power capacity. Hence, in practice, the configuration shown in FIG. 1 will not be employed. Hence, it is required to avoid the latch-up in a system having two power supply systems as shown in FIG. 4.